1. Field of the Invention
The present invention relates to a corona charging type charging device and an image forming apparatus.
2. Description of the Related Art
In electrophotographic image forming apparatuses, such as a copying machine, a printer, and a facsimile, an image is formed on recording paper by uniformly charging a surface of a photoreceptor, which serves as an image carrier and has a photosensitive layer including a photoconductive material formed on the surface, with electric charges applied, forming an electrostatic latent image corresponding to image information by various image creating processes, developing the electrostatic latent image with a developer supplied from a developing section and containing toner to thereby form a visual image, transferring the visual image onto a recording material such as paper, and heating and pressing the recording material using a developing roller in order to fix the visual image to the recording paper.
In such an image forming apparatus, a charging device is typically used to make the surface of the photoreceptor, electrically charged. Charging devices are classified into: a non-contact type charging device including a discharge electrode used to perform corona discharge onto a photoreceptor, a grid electrode that is an electrode which is provided between a surface of the photoreceptor and the discharge electrode as needed and which controls the amount of electric charges applied to the surface of the photoreceptor by the discharge electrode and the charged potential of the surface of the photoreceptor, and a support member that supports the discharge electrode and the grid electrode; and a contact type charging device that uses a charged roller or a charging brush. Since the grid electrode can control the charged potential of the surface of the photoreceptor almost correctly, the non-contact type charging device in which the grid electrode is provided is mainly used in an image forming apparatus that needs high speed.
As examples of the discharge electrode which performs corona discharge, a wire electrode, a metal plate electrode (hereinafter, referred to as a ‘needle-like electrode’) having a plurality of needle-like portions, and the like are used. Among these examples, the needle like electrode which is advantageous in that its number of constituent components is small, it has a long life, the amount of ozone generated is small, and few failures occur since an open circuit does not easily occur is preferably used. The needle-like electrode is manufactured by etching a metal plate mainly formed of an iron-based metal material, such as stainless steel, to form a plurality of needle-like portions. The iron-based metal material, such as stainless steel, which is a material of the needle-like electrode has high durability but has disadvantages that the iron-based metal material is easily oxidized by the moisture under a high-humidity environment, a nitrogen oxide or ozone generated by corona discharge at the time of charging operation, and the like.
In order to solve such a problem, it was proposed to provide a cover layer on a needle-like electrode surface. For example, Japanese Unexamined Patent Publication JP-A 2004-4334 proposes a charging device including a needle-like electrode with a surface on which a cover layer, which is formed of gold, platinum, copper, nickel, or chromium by plating processing, is formed. The needle-like electrode is manufactured by methods, such as etching processing and precision press. A processed section of the needle-like electrode manufactured by such methods is not sufficiently smooth and accordingly, fine uneven portions are generated. Such fine uneven portions also remain in the needle-like electrode having a cover layer formed by plating processing that is disclosed in JP-A 2004-4334. In the needle-like electrode with fine uneven portions, the applied voltage controllability which is the controllability of a voltage applied to the needle-like electrode is degraded, which disturbs the balance of corona discharge. This makes the charged potential of the surface of the photoreceptor uneven. In addition, contaminants, such as fine toner, easily adhere to the fine uneven portions formed in the needle-like electrode. That is, the needle-like electrode disclosed in JP-A 2004-4334 is disadvantageous in that the applied voltage controllability of the needle-like electrode is further degraded since contaminants, such as fine toner, adhere to the needle-like electrode by long-terra use, which makes the charged potential of the surface of the photoreceptor more uneven.
On the other hand, a wire grid electrode formed of stainless steel or tungsten, a perforated plate-like grid electrode having a plurality of through holes formed in a metal plate (grid base) formed of stainless steel, and the like are used as the grid electrode. Similar to the case of the needle-like electrode, the iron-based metal material, such as stainless steel, which is a material of the grid electrode has high durability but has disadvantages that the iron-based metal material is easily oxidized by the moisture under a high-humidity environment, a nitrogen oxide or ozone generated by corona discharge at the time of charging operator, and the like. In addition, in the case of using the grid electrode for a long period of time, the use under the high-humidity environment, the contact with ozone or a nitrogen oxide, and the like are unavoidable. For this reason, in the grid electrode formed of a metallic material, such as stainless steel, corrosion, such as rust, occurs due to the moisture in the air, ozone, or a nitrogen oxide. This lowers the durability. Furthermore, since the controllability of the charged potential of the surface of the photoreceptor becomes insufficient, the charged potential of the surface of the photoreceptor becomes uneven. Accordingly, a desired charged potential cannot be stably applied to the surface of the photoreceptor all the time. In addition, also in the case of the wire electrode, there is a problem that corrosion, such as rust, occurs in the grid electrode due to the ozone generated by corona discharge and the charged potential of the surface of the photoreceptor becomes uneven, which should be solved.
In view of the above problems in the charging device, for example, Japanese Unexamined Patent Publication JP-A 11-40316 (1999) proposes a charging device including a wire electrode, which is provided to extend inside a shielding case having one opened surface, and a plate-like grid electrode disposed between the wire electrode and a photoreceptor, the plate-like grid electrode being obtained by forming a nickel-plated layer with a thickness of about 1 μm on a surface of a perforated plate made of stainless steel and then forming a gold-plated layer with a thickness of about 0.3 μm on the nickel-plated layer. Since the gold-plated layer is formed on the nickel-plated layer in the plate-like grid electrode disclosed in JP-A 11-40316, the gold-plated layer does not peel easily and the corrosion resistance and the controllability of the charged potential of the surface of the photoreceptor are relatively good. However, since it is necessary to perform plating processing twice, that is, nickel plating processing and gold plating processing, in order to manufacture the plate-like grid electrode, the manufacturing process is complicated, which causes a problem that the cost is increased.
Furthermore, in order to sufficiently realize the above preferable characteristics in the plate-like grid electrode, the thickness of the gold-plated layer needs to be set to 0.3 μm or more. In addition, since the plate-like grid electrode is a relatively large member having almost the same size as the photoreceptor, the plated layer should be made thick. Accordingly, the amount of gold used also increases inevitably. Such heavy use of gold raises the price of a charging device and the price of an image forming apparatus, which impairs the versatility-based on a relatively-low price that is one of the advantages of the image forming apparatus. Therefore, a charging device which is excellent in the durability and the controllability of the charged potential of the surface of the photoreceptor without using an expensive material, such as gold, is desired.
Furthermore, Japanese Unexamined Patent Publication JP-A 2001-166569 proposes a charging device including a wire electrode and a plate-like grid electrode which is obtained by forming a gold-plated layer directly on a surface of a metal plate made of stainless steel using an electroplating method using a pulse current. Since the gold-plated layer does not peel easily either similar to the plate-like grid electrode disclosed in JP-A 11-40316, the corrosion resistance is high and the controllability of the charged potential of a surface of a photoreceptor is also good. However, in the plate-like grid electrode, there is also the same disadvantage as in the charging device disclosed in JP-A 11-40316 since it is necessary to set the thickness of the gold-plated layer to be 0.3 μm or more.
In addition, when analyzing the phenomenon in which corrosion, such as rust, occurs in the grid electrode, FTIR analyses (nitrate ion: NO3−) proved that a nitrogen oxide was generated on the electrode surface by using contaminants, such as fine dust, fine toner and its additives, and moisture adhering to the electrode surface, as a core. That is, the nitrogen oxide is generated on the electrode surface due to the contaminants adhering on the surface of the grid electrode, which causes the corrosion, such as rust. Since this degrades the charged potential controllability of the grid electrode, the charged potential of the surface of the photoreceptor becomes uneven.